Ask the Headmaster a yes or no question, and roll a test of Spellcraft. If you pass, the Headmaster will answer yes or no truthfully.
The Headmaster may deign to provide additional information.
If you fail the test, the Headmaster may remain silent, or answer maybe.
If you critically fail, the Headmaster may lie.

DC 10: Characters have all the pieces
DC 12: Characters are missing a piece or two
DC 15: Characters are missing a few pieces
DC 20: Characters are missing a lot
DC 30: Characters are completely clueless